This application relates to a simulator, particularly an apparatus and method of simulating or evaluating joint implants or artificial joints, for example, knee, spine, hip, and ankle joints, under simulated loading conditions. More particularly, the application relates to a mechanical simulator that is simple, accurate, and robust. However, features of the apparatus and method may be applicable to related uses that encounter the same or similar issues.
Artificial joints are prevalent and under continuous development and improvement. Just as important is the ability to test or simulate the artificial joint or implant under conditions that impose various loads representative of end use of the implant.
Commercially available simulators are relatively complex. For example, these units are typically servo-hydraulic units. However, these simulators are very expensive. Moreover, the servo-hydraulic simulators suffer drawbacks associated with their design. For example, the servo controls are electronic in nature and the lack of synchronous operation over an extended period of time can adversely impact test results. That is, separate servo controls are typically provided for separate motion of different axes. Trying to coordinate the separate motions over a large number of cycles (on the order of one to ten million cycles, for example) leads to the problem of the separate motions becoming out of synchronization.
As a result of the synchronization issue, or in an attempt to simplify and reduce the cost of the simulator, these known servo-hydraulic simulators often approximate the results of tests on specimens by adding the results from one set of motions to the other. For example, an implant may be exposed to cycling along one axis such as ten million cycles and then on a second axis in which the specimen has undergone another ten million cycles. The individual results are then mathematically superimposed in an effort to estimate the combined effects of the individual evaluations.
Still another issue with known simulators is that they are not as robust or rugged as desired. Multiple input sources are more prone to problems and not as desired as a single drive shaft.
Accordingly a need exists in the industry for a simulator that is accurate, robust, and versatile. The simulator must be able to provide a number of different types of motion, and able to maintain the desired synchronous movement even after an extended number of cycles. The ability to vary the different types of motion, e.g., vary the aspect ratio, is also a desired feature. There is also the need to expose the test specimen to static, compression, and dynamic forces while allowing motions about three orthogonal axes in a saline or bovine serum that approximates the end use of an implant, which may include the ability to vary the temperature at which the specimen is subjected. Further, the cost of the simulator must be reasonable in order to gain acceptance in the industry.